Problem: $ D = \left[\begin{array}{rrr}1 & 2 & 2 \\ 0 & 2 & 1 \\ 0 & 2 & 2\end{array}\right]$ What is $ D^{-1}$ ?
$ = \left[\begin{array}{rrr}1 & 0 & -1 \\ 0 & 1 & -\frac{1}{2} \\ 0 & -1 & 1\end{array}\right]$